To determine the moisture content in a sample, the sample is dried and the weight of the sample before and after the drying process is determined manually. Due to the extensive amount of work involved, this method is very expensive as well as error-prone.
In some cases, the weight loss can also be measured during the drying process. In a given sample, the decrease in weight is a function of the temperature, the length of the drying time, and the conditions in the test compartment, and it conforms to a weight-versus-time curve which asymptotically approaches the dry weight of the sample. The curve for the given sample is determined by comparative experiments and can be expressed mathematically through an approximation formula. A measuring instrument for gravimetric moisture determination which is appropriately equipped with available electronic technology can compute the moisture content of a sample based on the measured parameters of the aforementioned curve and based on the length of the drying time and indicate the result on a display unit. With this method, the substance to be dried no longer needs to be totally desiccated; it is sufficient to determine the coordinates of two measurement points in the weight-versus-time diagram.
As has already been mentioned at the beginning, the weight change of a sample is substantially a function of the temperature, the length of the drying time, and the conditions in the test compartment. Especially the stringent requirements imposed on the test compartment are setting a limit to the accuracy of the commercially available instruments.
The term “test compartment” in the present context means a space which is enclosed by the housing of the measuring instrument and which can be opened in order to insert or remove a sample. Also arranged inside the test compartment are a sample receiver and a means to heat the sample. The sample receiver is connected to a gravimetric measuring instrument.
Normally, the sample is spread in a thin layer onto a flat sample receiver, for example a sample tray. For a uniform heating of the sample, the sample tray is preferably positioned so that its flat area is horizontal and parallel to the planar area occupied by the sample-heating means.
As a means for heating the sample, a variety of radiation sources are used, such as heat radiators, microwave generators, halogen and quartz lamps. As was found in experiments, the type of radiation source being used and the way it is arranged in the test compartment are among the primary causes for inaccurate measurement results in existing gravimetric moisture-determination instruments.
A gravimetric moisture-determination instrument of the aforementioned type is disclosed in commonly-owned U.S. Pat. No. 5,485,684, which issued on 23 Jan. 1996 to Florian, et al. In this instrument, the sample substance is put on the weighing pan while the latter is outside of the gravimetric moisture-determination instrument. To do this, the balance is pulled out of the housing of the measuring instrument on a sliding carrier like a drawer. For a radiation source a ring-shaped halogen lamp is used which is located above the sample receiver when the instrument is in its operating condition.
One of the possibilities of preventing a thermal decomposition of the sample is to use a microwave generator as radiation source, as is disclosed in U.S. Pat. No. 6,521,876 B2, issued on 18 Feb. 2003 to Jennings, et al. One drawback of microwave heating is that samples with a non-homogeneous moisture distribution will also be non-homogeneously heated. The volatile components escaping from the heated areas of the sample, in particular moisture in the form of water vapor, can partially condense in the cold areas of the sample, so that there is a tendency for the moisture to first distribute itself within the sample before being driven out of the sample. The timing errors which occur as a result of this impose a limit on the accuracy that can be achieved in an analysis according to the calculation method described above. As an alternative to using the calculation method, one is only left with the choice to drive out all of the moisture contained in the sample. However, the smaller the amount of moisture that is present in the sample, the less heat is developed. Utensils such as weighing receptacles of a plastic material in which microwaves can generate vaporizing heat often have in themselves an inherent moisture content or an affinity for moisture, so that instead of a drying process an exchange of moisture between the sample and the weighing receptacle can take place. A locally concentrated overheating can likewise occur in weighing receptacles of a plastic material, in which case the plastic material can break down and the loss of mass due to escaping volatile decomposition products or due to substance being sublimated off the weighing receptacle can be measured erroneously as a weight loss of the sample.
For the reasons that have just been explained, it is hardly possible to determine an absolute value for the moisture content with a gravimetric moisture-determination instrument. For a more accurate determination of the moisture content of a substance, the known Karl Fischer titration method is therefore still in use. This method is very labor-intensive, prone to user errors, and expensive.
It is therefore the object to provide in a gravimetric moisture-determination instrument of the kind mentioned in the introduction having a test compartment with improved test conditions, in which the moisture content of a sample can be determined more precisely.